1. Field of the Invention
This invention relates generally to a shutter for a disk cassette, and particularly, a shutter to be most applicable to a floppy disk cassette and made of synthetic resin, and also, relates to a heating device used for a production of the same.
2. Description of the Related Art
As disclosed, for example, in U.S. Pat. No. 4,698,714, a disk cassette, such as a floppy disk cassette has a flat square case 3 made of synthetic resin, which comprises an upper half 1 and a lower half 2 as illustrated in FIGS. 7-9. Within the case 3, a disk-like recording medium, such as a floppy disk 4 is rotatably accommodated. Near the front side 3a of the case 3, the upper half 1 and the lower half 2 have respective rectangular head insertion openings 5 paired with each other. Further, at the center of the lower half 2, a circular turntable insertion opening 6 is provided.
A shutter 9 comprising a pair of upper and lower rectangular shutter plates 8 and a connecting plate for joining the plates 8, and being shaped into a U-form in section is slidably secured to cover a part, near to the front side 3a, of the case 3. In the event, the upper and lower shutter plates 8 are fitted in respective stepped portions 10 formed in the outer surfaces of the upper half 1 and the lower half 2, and is arranged in parallel with the respective surfaces of the upper half 1 and the lower half 2. The connecting plate 7 is also arranged in parallel with the front side 3a of the case 3, and the shutter 9 is movable in a direction of arrows a, b along the front side 3a between its shut position indicated in solid lines and its open position indicated in dot-dash lines in FIG. 7.
When the shutter 9 is moved into the shut position, rectangular openings 11 disposed in the respective upper and lower shutter plates 8 are not coincident with the respective head insertion openings 5 at all, so that the pair of head insertion openings 5 are closed, and when the shutter 9 is moved into the open position, the pair of openings 11 of the shutter 9 are coincident with the respective head insertion openings 5, so that the pair of head insertion openings 5 are opened. It is noted that the shutter 9 has been resiliently biased into the shut position along the direction of arrow a by the force of a spring (not shown).
There are two kinds of shutters 9, one is made of a stainless steel sheet by the help of press working, and the other is made of synthetic resin. Further, there are two kinds of production methods to make the shutter 9 of the synthetic resin.
One method of producing the shutter 9 of the synthetic resin is such that a synthetic resin plate 13 having a thickness T of less than 0.3 mm and made of polypropylene, vinylchloride or the like is press-worked, then horizontally mounted on a first jig 14 as shown in FIG. 5A, and after sandwiching the central portion of the plate 13 between the first jig 14 and a heated second jig 15, both the end portions of the plate 13 are bent in a direction of arrow c with a pair of third jigs 16 as shown in FIG. 5B in order to obtain a shutter that is of U-shape in section.
The other one method of producing the shutter 9 of the synthetic resin is such that a molten synthetic resin, such as polyacetal, polypropylene, polyethylene or the like is injected into a U-shaped cavity 19 through an injection gate 20 shown in FIG. 6 to make a shutter that is of U-shape in section. It is noted that the cavity 19 is formed between metallic molds 17, 18 and has a gap G of less than 0.3 mm.
However, if the shutter 9 is made by means of bending the synthetic resin sheet 13 while the synthetic resin sheet 13 is heated, residual stresses are produced, after bending, in corners 9a shown in FIG. 5B, where the respective shutter plates 8 are connected to the connecting plate 7, because the outer side of the corner 9a is cooled at high speed, whereas the inner side thereof is cooled at low speed.
If the shutter 9 is made by means of injecting the synthetic resin into the metallic mold 17, 18, residual stresses are also produced in the corners 9a after molding, because the molten synthetic resin 21 flows at high speed and turns sharply along the corner portion 19a of the cavity 19, and further, the molten synthetic resin 21 that filled the corner portion 19a can not take much cooling time as compared with the one that filled plain portions 19b, 19c of the cavity 19.
The residual stresses existing in the corner 9a of the shutter 9 act as a force for transferring the pair of shutter plates 8 in a direction of arrow d in FIG. 5B, so that if the disk cassette is used at high temperature, the pair of shutter plates 8 of the shutter 9 is easily deformed in the direction of arrow d as indicated in dot-dash lines in FIG. 8. As the result, sometimes the disk cassette is difficult to draw from a recording and/or reproducing apparatus, and sometimes, the shutter drops out of the case 3.